In a constant speed operation device for an automobile or motorcycle, the automatic constant speed operation may be manually cancelled, but, for the sake of convenience, an automatic cancellation system is generally used in combination with such manual cancellation system. For instance, the system disclosed in Japanese Patent Publication No. 61-6032 of the present applicant includes a switch interlocking with the operation of a throttle grip mounted on a handle. The switch gives a cancellation command for cancelling the automatic constant speed operation when the throttle grip is operated in the decelerating direction. The system disclosed in the Japanese Patent Publication No. 52-1887 includes a switch interlocking with the operation of a clutch or brake.
However, the first of the above described prior art devices cancels the automatic constant speed operating condition according to a rotative position of the throttle grip irrespective of the vehicle operating speed, while the second requires operation of the cancelling means to manually drive the automobile. Therefore, it is impossible to obtain a constant speed operation control corresponding to engine performance. I.e., in an engine capable of generating a high torque even at low engine speeds, constant speed operation control may be maintained down to a very low vehicle speed. Accordingly, as far as constant speed operation control is permissible in correspondence with engine performance, it is desired to maintain automatic constant speed operation even after deceleration. On the other hand, it is also desired to cancel the automatic constant speed operation by a decelerating operation in case of an emergency or the like.